refactor(desktop): share the large browse dialogs' sizing

LargeDialogBounds computes the bounds the large browse dialogs open at:
nine tenths of the owner screen's working area, centered, never below the
dialog's minimum size. WinForms' DialogBounds now calls it with the
screen's working area, with unchanged behaviour.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Desktop.Tests/LargeDialogBoundsTests.cs

@@ -0,0 +1,28 @@
+namespace OpenNest.Desktop.Tests;
+
+public class LargeDialogBoundsTests
+{
+    [Fact]
+    public void Centered_NineTenthsOfTheWorkingArea()
+    {
+        var bounds = LargeDialogBounds.Centered(new PixelRect(0, 0, 1920, 1040), 800, 600);
+
+        Assert.Equal(new PixelRect(96, 52, 1728, 936), bounds);
+    }
+
+    [Fact]
+    public void Centered_OnASecondaryScreen()
+    {
+        var bounds = LargeDialogBounds.Centered(new PixelRect(1920, -200, 2560, 1400), 800, 600);
+
+        Assert.Equal(new PixelRect(1920 + 128, -200 + 70, 2304, 1260), bounds);
+    }
+
+    [Fact]
+    public void Centered_NeverBelowTheMinimumOrBeforeTheWorkingArea()
+    {
+        var bounds = LargeDialogBounds.Centered(new PixelRect(100, 50, 1000, 700), 1200, 900);
+
+        Assert.Equal(new PixelRect(100, 50, 1200, 900), bounds);
+    }
+}

OpenNest.Desktop/LargeDialogBounds.cs

@@ -0,0 +1,24 @@
+namespace OpenNest.Desktop;
+
+/// <summary>
+/// Sizing shared by the large browse dialogs (File &gt; Open, Drawing Library, Shape Library) so
+/// they open at the same size: a normal window nine tenths of the owner screen's working area,
+/// centered in it so its bottom row stays clear of the taskbar.
+/// </summary>
+public static class LargeDialogBounds
+{
+    /// <summary>
+    /// The bounds in <paramref name="workingArea"/> (screen pixels), never smaller than the
+    /// dialog's minimum size and never starting left of or above the working area.
+    /// </summary>
+    public static PixelRect Centered(PixelRect workingArea, int minimumWidth, int minimumHeight)
+    {
+        var width = System.Math.Max(minimumWidth, workingArea.Width * 9 / 10);
+        var height = System.Math.Max(minimumHeight, workingArea.Height * 9 / 10);
+        return new PixelRect(
+            System.Math.Max(workingArea.X, workingArea.X + (workingArea.Width - width) / 2),
+            System.Math.Max(workingArea.Y, workingArea.Y + (workingArea.Height - height) / 2),
+            width,
+            height);
+    }
+}

OpenNest/Forms/DialogBounds.cs

@@ -1,5 +1,6 @@
 using System.Drawing;
 using System.Windows.Forms;
+using OpenNest.Desktop;
 
 namespace OpenNest.Forms;
 
@@ -20,12 +21,10 @@ internal static class DialogBounds
     public static void FitToOwnerScreen(Form form)
     {
         var area = Screen.FromControl(form.Owner ?? form).WorkingArea;
-        var width = System.Math.Max(form.MinimumSize.Width, area.Width * 9 / 10);
-        var height = System.Math.Max(form.MinimumSize.Height, area.Height * 9 / 10);
-        form.Bounds = new Rectangle(
-            System.Math.Max(area.Left, area.Left + (area.Width - width) / 2),
-            System.Math.Max(area.Top, area.Top + (area.Height - height) / 2),
-            width,
-            height);
+        var bounds = LargeDialogBounds.Centered(
+            new PixelRect(area.X, area.Y, area.Width, area.Height),
+            form.MinimumSize.Width,
+            form.MinimumSize.Height);
+        form.Bounds = new Rectangle(bounds.X, bounds.Y, bounds.Width, bounds.Height);
     }
 }